This invention, relates to PVA compositions, useful in creping tissue paper. The desired properties of tissue paper, including softness, bulk, flexibility, and absorbency, are achieved by a process known as “creping”. Typically, paper creping takes place on a heated creping cylinder (often referred to as a “Yankee Dryer”) by the actions of a doctor blade. Creping is performed by transferring a wet paper sheet on to the creping cylinder's surface after it has been largely dewatered at a pressure roll nip. When first transferred, the non-water component of the sheet is about 35-90% of its mass. The sheet is further dried on the hot creping cylinder to achieve a non-water consistency of about 90-98% of its mass. The sheet then is removed from the creping cylinder by impact with the doctor blade. The mechanical action of the doctor blade results in a disruption of the fiber-fiber bonds in the sheet and forms microfold-structures within the sheet. These disruptions and microfold-structures produce many of the desired properties of tissue paper.
In order to crepe, the paper sheet must adhere to the surface of the creping cylinder. The adhesion is accomplished by the use of an adhesive coating applied to the creping cylinder. The most common adhesive coatings are synthetic polymers such as polyaminoamides, polyamides, polyamines, polyvinyl alcohols, polyvinyl acetates, polyacrylamides and polyethers. Various low molecular weight compounds are used to modify the coating properties. In order to function properly, an adhesive coating must have adequate: (i) uniformity, (ii) durability, and (iii) softness.
Uniformity of the coating is important for a few reasons. First, it affords consistent creping that affects the quality of the produced sheet. Also, it prevents direct exposure of the creping cylinder surface to the blade, which can lead to premature wear of the cylinder and the blade.
Durability is a characteristic of how well the coating remains on the creping cylinder's surface, particularly at the pressure roll nip. It is important because, if the coating is easily washed off, it exposes the creping cylinder and leads to excessive wear on the blade and the cylinder surface.
Softness is also important, because a coating that is too hard causes blade chatter. Blade chatter causes blade wear and non-uniform creping. Therefore, a coating which is uniform, durable, and soft is ideal.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made, or that no other pertinent information, as defined in 37 CFR §1.56(a), exists.